The production of plastic ammunition casings seeks, among other objectives, the reduction of weight and cost. A plastic ammunition casing, versus its equivalent in brass, can reduce weight up to 60% and, in the overall weight of the assembled ammunition, may represent a decrease of total weight of 20% on average.
The high cost of copper and market variability makes the price of the raw material have large oscillations, which does not happen so severely in plastics.
The use of plastic materials for the manufacture of ammunition casings is not a new idea. In the 1950's, U.S. Pat. No. 2,654,319 (ROSKE) and U.S. Pat. No. 2,862,446 (RINGDAL) proposed designs and methods for the manufacture of these components with polymeric materials.
From those early developments to the most recent attempts, as discussed in U.S. Pat. No. 6,845,716 (HUSSEINI, BYRON) and U.S. Pat. No. 7,213,519 (WILEY, REMBERT, LOFTIN), which were the start of companies like NATEC or POLITECH respectively, started from the idea that the casing head must be made of metal to ensure strength.
More recent proposals such as US20130014664 (PADGETT), US20120180688 (PADGETT) WO2012/947615 (DAVIES, COCO) or US20070261587 (CHUNG) also incorporate a metal bushing in the casing using different techniques.
However, all the above proposals, when applied to the manufacture of ammunition casings with shoulder, i.e., bottle shaped, in order to maximize the inner space, need several parts in different combinations to form the entire casing. The reason is the difficulty to release the undercut that forms the inner surface.
Considering the small size of the parts and the mechanical strength of the mold needed to hold the pressure used during the injection process, known techniques make it impossible to solve the undercut with mechanical movements. Then, to make a constant wall thickness along the external shape to maximize space for the powder, the body that contains the metal bushing and the part that contains the shoulder are injected separately and then bonded by welding or adhesive.
On one hand, the pressures which occur within the ammunition casing while shooting in conjunction with the temperature of the chamber, makes softer the weld zone, particularly when it remains for a long interval of time in the hot chamber before shot. On the other hand, the fact that the casing is composed of several pieces joined together does not guarantee the sealing and makes the cartridge more sensitive to suffer mechanical breakage.
What is need in the art is a method of making molded objects, such as ammunition casings, that enable the formation of a molded object without the problems noted above in the prior art. What is further need in the art is an apparatus capable of making molded objects, such as ammunition casings, that enable the formation of a molded object without the problems noted above in the prior art.